Pile Design and Construction Practice, Fifth edition

456 Miscellaneous piling problems

Figure 9.15Piling for bridges with spill-through abutments (a) Bank seat carried by piles driven
from completed embankment (b) Bank seat carried by columns with pile cap at original
ground level.

Horizontal earth and surcharge pressures on bridge abutments and wing walls are resisted
much more efficiently by raking piles than by vertical piles. However, rakers provide a high
degree of rigidity to the foundation in a horizontal direction which may require designing for
at-rest earth pressures rather than the lower active pressures which depend on permitted
yielding of the retaining structure. At-rest pressures are likely to be operating when the top
of the abutment is strutted by the bridge deck as well as being restrained at the toe by the
rows of raking piles.
The simple and computer-based methods of determining individual pile loads in groups
of vertical and raking piles carrying a combination of vertical and lateral loads were
described in Section 6.5. Hambly(9.22)has pointed out the desirability of varying the angle
of rake in order to avoid concentration of load on the bearing stratum, although some design-
ers consider that because of ‘buildability’considerations, raking piles should only be used
when absolutely necessary. The choice of bored or driven piles for combined vertical and
raked pile groups should take account of the need to install casing and reinforcement and
place concrete in a raking bored pile compared to achieving the designed set with a reduced
efficiency when driving on a rake.
In the case of bridges with spill-through abutments and embanked approaches the piles
supporting the bank seats are best installed from the surface of the completed embankment
(Figure 9.15a). In this way the drag-down forces from the settling embankment and any
underlying compressible soils are carried preferably by vertical piles. The drag-down force
can be minimized by using slender sections in high strength materials. If the piles are
constructed at ground level with the bank seat supported on columns erected on a pile cap,
the latter will act as a ‘hard-spot’attracting load from the embankment fill (Figure 9.15b).
Unless precautions are taken the higher loading on the piles supporting the low level pile
cap will result in greater tendency for them to settle relatively to the piles supporting the
adjacent bridge pier with consequent differential movement in the bridge deck.
Vertical piles as shown in Figure 9.15b are preferable to rakers for supporting bridge
abutments constructed on ground underlain by a soft deformable layer, whether or not the
abutments are of the spill-through type or in the form of vertical retaining walls and inclined

Bank seat

Embankment

Dragdown

force on

pile shaft

Compressible soil

Original

ground level

Compressible soil

Columns

Dragdown force

reduced by pile

cap

Pile cap

(a) (b)

Load on pile

cap from

settling fill